A fair point that occurred to me as soon as I pressed send.
But given the aim is to reduce how many smokers there are in the first place (something we've had some success in doing already so it's not a nonsensical idea) I think a decent portion of the money would just go elsewhere in the...
Presumably, the money that people currently spend on cigarettes will be spent on something, it's not like smokers will just chuck £10 notes into a canal instead.